Photographic processes have been essential to the microelectronics industry. Accurate, reproducible lines with widths as small as a half micron are routinely produced in monolithic integrated circuits by exposing photosensitive polymers to ultraviolet light through masks which have been photographically fabricated. Submicrometer patterns have been realized using materials that are sensitive to electron beams or x-rays, but such processes are relatively more expensive to implement than systems utilizing light.
Unfortunately, however, when a beam of light is transmitted through an aperture, diffraction causes the light to spread into the dark regions surrounding the beam. An imaging optical system such as that used in photolithography can collect much of the transmitted light and thereby form an image of the aperture, but the finite numerical aperture of any such system causes the image to spread nonetheless. Constructive interference between the light diffracted by adjacent apertures in a transmission mask increases the intensity of light between the apertures, thereby reducing resolution of the optical system. This diffraction phenomenon limits the minimum feature sizes of integrated circuits fabricated using optical lithography.
If it can be arranged that the light waves transmitted through adjacent mask apertures are 180.degree. out of phase, the resultant destructive interference minimizes the intensity between the images engendered by the apertures. This situation may be brought about by covering one of the mask apertures with "phase-shifting elements", i.e., transmissive material of appropriate thickness and index of refraction. Any given photolithographic system will project the images of such a phase-shifting mask with better resolution and higher contrast than it would the images of a corresponding mask lacking phase-shifting elements.
The improvement in resolution afforded may be appreciated by referring to FIGS. 1A and 1B. FIG. 1A shows a conventional transmission mask M having a pair of closely spaced transparent regions T1. In the system of FIG. 1A an optical source (not shown) is provided to illuminate the mask M from above. The bell-shaped distributions indicated by the dotted lines below the mask M represent the wavefront amplitudes which would be developed in plane P1 were the mask M to include only one of the transparent regions T1. The intensity of the resultant image simply corresponds to the squared magnitude of the amplitude. As is indicated by the solid line below the mask in FIG. 1A, when the mask M includes both of the apertures T1 the constructive interference between the wavefronts diffracted by each aperture leads to difficulty in resolving the separate image features.
Referring to FIG. 1B, there is shown a phase-shifting mask PM having a transparent region T2 and a phase-shifting transparent region T3. The phase-shifting mask PM includes a phase-shifting element PS in alignment with transparent region T3, which shifts the phase of the wavefront propagating therethrough by 180 degrees. As is evidenced by the trough in the solid line representation of wavefront intensity below the mask PM, such phase-shifting leads to at least partial cancellation of the optical energy diffracted into the area between the regions T2 and T3. The associated improvement in resolution is indicated by the greater separation between the peaks in the solid line, each of which correspond to a feature in the resultant image.
Despite the improved resolution offered by phase-shifting masks advantages, widespread use thereof has been impeded by the difficulty in determining the correct pattern of phase-shifting elements corresponding to a particular microelectronic circuit.
Existing approaches to the design of phase-shifting masks tend to be based either on intuition or on brute force methodologies such as trial and error. Such heuristic techniques tend to be of assistance only in connection with relatively simple circuit patterns.
Accordingly, it is an object of the present invention to provide a procedure enabling the systematic production of phase-shifting masks for integrated circuits of complex geometry.